Turbochargers are well known devices for supplying air to the intake of an internal combustion engine at pressures above atmospheric pressure (boost pressures). A conventional turbocharger essentially comprises an exhaust gas driven turbine wheel mounted on a rotatable shaft within a turbine housing connected downstream of an engine exhaust manifold. Rotation of the turbine wheel rotates a compressor wheel mounted on the other end of the shaft within a compressor housing. The compressor wheel delivers compressed intake flow to the engine intake manifold to increase power and fuel efficiency over what is possible with a normally aspirated engine.
One known approach to improving turbocharging efficiency and reducing emissions for an engine with a wide speed/load range is to provide a sequential two stage turbocharging system, comprising one relatively small high pressure turbocharger and another relatively large low pressure turbocharger. The turbochargers are arranged in series so that exhaust from the engine flows first through the smaller turbine of the high pressure turbocharger and then through the larger turbine of the low pressure turbocharger. The compressors of the two turbochargers are also arranged in series, with intake air flowing first through the relatively large compressor of the low pressure turbocharger and then through the relatively small compressor of the high pressure turbocharger. Compression of the intake flow raises the temperature of the intake flow, which can then be cooled via a heat exchanger to increase the density of the intake flow in the intake manifold. This also reduces undesirable effects of auto-ignition or knock in the combustion chamber.
One problem with multi-stage turbocharger systems concerns the injection of fuel. When fuel is injected upstream of the first low pressure compressor, extra work is required since the air and fuel mixture, also known as the charge flow, is compressed in each of the compressor stages. When fuel is injected downstream of the last or high pressure compressor, then inadequate mixing of the fuel with the air of the intake flow may result in uneven fuel distribution between combustion chambers of the engine's cylinders. Furthermore, the quality of combustion in the combustion chambers may suffer from lack of homogeneity of the charge flow distributed to the cylinders. Therefore, there remains room for further improvements in this technology area.